Q&A: Asphalt Shingles on Unvented Roofs

Q.Our insulation subcontractor
advises us to install dense-pack cellulose in the
rafter bays over a cathedral ceiling, without
ventilation baffles, and the local building inspector
approves. If we don’t ventilate under the roof
sheathing, what happens to our asphalt shingle
warranty?

A.Contributing editor Ted
Cushman responds: With few exceptions,
installing shingles on an unventilated roof deck
will void the warranty. The fact that your local
building department okayed the installation
probably won’t help: As one warranty (from
the Canadian firm EMCO, which makes Esgard
shingles) states, "Where local building codes have
specific [ventilation] requirements that differ
from the National Building Codes, the more
stringent requirement must be followed."

In fact, as you might guess from that quote,
just having the roof ventilated may not be good
enough. The shingle maker has to agree that the
ventilation is up to its standards. If the warranty
excludes installations with "improper ventilation,"
that language may be enough to deny a claim.

Unfortunately, shingle warranties provide little
protection, whether or not your roof deck is
ventilated. Almost no roof is put on perfectly; and
if you’ve deviated from the
manufacturer’s instructions in any way,
your claim can be denied. In the end, it comes down
to trust: If manufacturers want to back the product
up, they will, and if they don’t, they
won’t. You’ve got to decide if
you want to trust them. And even if a warranty is
honored, the money you’ll get
won’t begin to cover costs like tearoff,
disposal, or labor.

The real question is, how will the shingles hold
up on an unventilated roof deck? They might give
out a little sooner than shingles on a ventilated
roof, but probably not enough to notice. The most
important factor in longevity isn’t the
level of ventilation; it’s the quality of
the shingle. The better brands of fiberglass
shingles are the ones labeled as passing ASTM
Standard D-3462. If you start with a good shingle,
what you lose in shingle life (if anything) you
probably more than earn back in energy savings from
the added insulation thickness.

At least one manufacturer, CertainTeed, will
honor its warranty on an unventilated roof deck,
although for a reduced term of ten years (prorated
from year one, and with no wind speed rating).
CertainTeed has funded a lot of research on the
causes of shingle failure, including some long-term
studies at university sites in three different
climates. In these studies, shingles were applied
on ventilated and unventilated cathedral roofs side
by side. The research indicates that high
temperatures do cut the lifespan of shingles, but
only marginally; and it shows that roof ventilation
doesn’t have much of an effect on shingle
temperature anyway (shingle color and roof
orientation are more important).

However, ventilation affects not only shingle
temperature but also the level of moisture in the
roof assembly, as well as the melting and
refreezing of snow on the roof. Since these factors
can affect how a shingle ages, it may be reasonable
for manufacturers to limit or exclude warranty
coverage on unventilated roofs.